Over time, the harmful components of nitrogen oxides (NOx) have led to atmospheric pollution. NOx is contained in exhausted gases such as from internal combustion engines (e.g., automobiles and trucks), from combustion installations (e.g., power stations heated by natural gas, oil, or coal), and from nitric acid production plants.
Various methods have been used in the treatment of NOx-containing gas mixtures. One type of treatment involves catalytic reduction of nitrogen oxides. There are two processes: (1) a nonselective reduction process wherein carbon monoxide, hydrogen, or a lower hydrocarbon is used as a reducing agent, and (2) a selective reduction process wherein ammonia or ammonia precursor is used as a reducing agent. In the selective reduction process, a high degree of removal with nitrogen oxide can be obtained with a small amount of reducing agent.
The selective reduction process is referred to as a SCR process (Selective Catalytic Reduction). The SCR process uses catalytic reduction of nitrogen oxides with ammonia in the presence of atmospheric oxygen with the formation predominantly of nitrogen and steam:4NO+4NH3+O2→4N2+6H2O (standard SCR reaction)2NO2+4NH3→3N2+6H2O (slow SCR reaction)NO+NO2+NH3→2N2+3H2O (fast SCR reaction)
Catalysts employed in the SCR process ideally should be able to retain good catalytic activity over the wide range of temperature conditions of use, for example, 200° C. to 600° C. or higher, under hydrothermal conditions. Hydrothermal conditions are often encountered in practice, such as during the regeneration of a soot filter, a component of the exhaust gas treatment system used for the removal of particles.
Molecular sieves such as zeolites have been used in the selective catalytic reduction (SCR) of nitrogen oxides with a reductant such as ammonia, urea, or a hydrocarbon in the presence of oxygen. Zeolites are crystalline materials having rather uniform pore sizes which, depending upon the type of zeolite and the type and amount of cations included in the zeolite lattice, range from about 3 to 10 Angstroms in diameter. Zeolites having 8-ring pore openings and double-six ring secondary building units, particularly those having cage-like structures have recently found interest in use as SCR catalysts. A specific type of zeolite having these properties is chabazite (CHA), which is a small pore zeolite with 8 member-ring pore openings (˜3.8 Angstroms) accessible through its 3-dimensional porosity. A cage like structure results from the connection of double six-ring building units by 4 rings.
Metal-promoted zeolite catalysts including, among others, iron-promoted and copper-promoted zeolite catalysts, for the selective catalytic reduction of nitrogen oxides with ammonia are known. Iron-promoted zeolite beta has been an effective commercial catalyst for the selective reduction of nitrogen oxides with ammonia. Unfortunately, it has been found that under harsh hydrothermal conditions, for example exhibited during the regeneration of a soot filter with temperatures locally exceeding 700° C., the activity of many metal-promoted zeolites begins to decline. This decline is often attributed to dealumination of the zeolite and the consequent loss of metal-containing active centers within the zeolite.
Metal-promoted, particularly copper promoted aluminosilicate zeolites having the CHA structure type and a silica to alumina molar ratio greater than 1, particularly those having a silica to alumina ratio greater than or equal to 5, 10, or 15 and less than about 1000, 500, 250, 100 and 50 have recently solicited a high degree of interest as catalysts for the SCR of oxides of nitrogen in lean burning engines using nitrogenous reductants. This is because of the wide temperature window coupled with the excellent hydrothermal durability of these materials, as described in U.S. Pat. No. 7,601,662. Prior to the discovery of metal promoted zeolites described in U.S. Pat. No. 7,601,662, while the literature had indicated that a large number of metal-promoted zeolites had been proposed in the patent and scientific literature for use as SCR catalysts, each of the proposed materials suffered from one or both of the following defects: (1) poor conversion of oxides of nitrogen at low temperatures, for example 350° C. and lower; and (2) poor hydrothermal stability marked by a significant decline in catalytic activity in the conversion of oxides of nitrogen by SCR. Thus, the invention described in U.S. Pat. No. 7,601,662 addressed a compelling, unsolved need to provide a material that would provide conversion of oxides of nitrogen at low temperatures and retention of SCR catalytic activity after hydrothermal aging at temperatures in excess of 650° C.
Even though the catalysts described in U.S. Pat. No. 7,601,662, exhibit excellent properties, there is always a desire for improved performance in extended or different temperature windows. Thus, one of the challenges of meeting current governmental (for example, Euro 6) NOx regulations is the improvement of low temperature performance while maintaining the high temperature performance of the SCR catalyst. Accordingly, an SCR catalyst is needed that achieves a balanced performance for both low and high temperatures.